It is often a problem in data processing systems to perform maintenance functions without inconveniencing the system users by removing the system from service. For this reason, it is common practice to perform routine maintenance during evening hours when the system is lightly loaded so that a minimum number of users will be inconvenienced. This practice is effective for installations in which maintenance can be scheduled and deferred until the evening hours when the system is less utilized than it is during daytime hours. This practice is not suitable for installations in which the system is heavily utilized 24 hours a day. It is also not suitable for emergencies which may occur at any time and which must be promptly remedied to restore the system to full operating efficiency.
Some maintenance can be performed while a machine remains operational by removing and replacing a non-critical defective element. However, there are certain critical elements which cannot be removed without affecting system operation. When trouble is detected in these critical elements, the entire system must be shut down so that the element can be replaced. This, of course, is undesirable, since it inconveniences the system users and keeps the system out of service until the defects are remedied.
An example of a critical system element is the system data buses and their connected circuits. Typically, each data bus is connected to a plurality of logic circuit cards which receive the signals applied to the bus by a driver circuit. Circuitry on the logic cards responds to the bus data signals and controls the system function for which each logic card is responsible. Defects can occur in these logic cards and normal maintenance procedure is to remove a defective card and replace it with a new one.
Data buses are typically transmission lines having precise impedance matching requirements which must be met so that reflected signals and standing waves do not appear on the busses. These signals could be reflected back to the bus drivers and destroy them as well as other portions of the system to which the drivers are connected. The number of logic cards that are connected to the bus must also be carefully controlled so that the impedance matching requirements of the bus are met. A typical data bus is connected to one or more logic cards and the end of the bus is connected to a termination resistor network which matches the characteristic impedance of the bus to minimize the generation of destructive standing waves. This impedance matching requirement precludes the addition or removal of a logic card to or from a data bus while the data bus remains active. For this reason, it is not an acceptable procedure to remove a defective card from a bus and replace it with a new card without withdrawing the system from service.
It can therefore be seen as is often a problem in data processing systems to perform maintenance functions without first removing the system from service.